Long-term observations on plasma antidiuretic hormone levels during and after heat stress.

Eleven healthy subjects aged 18--26 years underwent intermittent heat stress in a sauna bath. Blood samples were taken immediately before, during and at various intervals after heat exposure for the measurement of antidiuretic hormone, osmolality and percentage change of plasma volume. Antidiuretic hormone was increased during, immediately after and 90 min after heat stress. Three hours after the heat stress period antidiuretic hormone returned to control values in spite of a significant increase in osmolality and a significant decrease in plasma volume. These results imply that other factors than ADH are responsible for the long-term homeostasis of water balance and plasma volume.